Coupled NOx storage and reduction and selective catalytic reduction using dual-layer monolithic catalysts

Abstract

Lean reduction of NOx (NO and NO2) was studied using monolith-supported catalysts consisting of a layer of a metal-exchanged (Fe, Cu) zeolite (ZSM-5) selective catalytic reduction (SCR) catalyst deposited on top of a Pt/Rh/BaO/CeO2 lean NOx trap (LNT) catalyst. During periodic switching between lean and rich feeds, the LNT layer reduces NOx to N2 and NH3. The SCR layer traps the latter, resulting in additional NOx reduction. The dual-layer catalysts exhibited high N2 selectivity and low NH3 selectivity over the temperature range of 150–300°C. The NOx conversion was incomplete due to undesired side reactions in the LNT layer, such as NH3 oxidation to N2O at low temperature and NOx at high temperature. The NOx conversion and N2 selectivity of the Cu-exchanged ZSM-5 was higher than that of the Fe-exchanged ZSM-5. This was due to a higher low temperature SCR activity and a higher NH3 storage capacity on the Cu-zeolite. The dual-layer catalyst had a higher NOx conversion than the LNT catalyst below 300°C and a higher N2 selectivity over the entire temperature range when H2O and CO2 were present in the feed. The NOx storage capacity and NH3 generation increased upon addition of CeO2 to the LNT layer below 250°C. It also led to increased NH3 oxidation at high temperatures. The addition of ceria mitigated the undesirable migration of Pt between the LNT and SCR layers. Hydrothermal aging had a smaller effect on dual-layer catalysts containing ceria.